Metals from Groups I and II of the periodic table are commonly used to initiate the polymerization of monomers into polymers. For example, lithium, barium, magnesium, sodium, and potassium are metals that are frequently utilized in such polymerizations. Initiator systems of this type are of commercial importance because they can be used to produce stereo regulated polymers. For instance, lithium initiators can be utilized to initiate the anionic polymerization of isoprene into synthetic polyisoprene rubber or to initiate the polymerization of 1,3-butadiene into polybutadiene rubber having a derived microstructure.
The polymers formed in such polymerizations are terminated with the metal used to initiate the polymerization and are sometimes referred to as living polymers. They are referred to as living polymers because the polymer chains which are terminated with the metal initiator continue to grow or live until all of the available monomer is exhausted and/or until the polymerization is terminated by the addition of an agent that “kills” the living polymer chain ends, such as an alcohol. Polymers that are prepared by utilizing such metal initiators normally have structures which are essentially linear and normally do not contain appreciable amounts of branching.
Such rubbery polymers that do not contain appreciable amounts of branching have certain drawbacks in that their flow characteristics at room temperature are extremely high and in that their tensile strength and tear resistance in the unvulcanized state are very poor due to less chain entanglement among their molecular chains. Due to these characteristics, the processing of such rubbery polymers prior to vulcanization is sometimes difficult. In order to improve the cold flow characteristics, tensile strength, and tear resistance of such unvulcanized rubbers they are often crosslinked prior to processing and subsequent vulcanization. Such metal terminated rubbery polymers can be crosslinked by treatment with compounds that contain two or more vinyl groups, such as divinylbenzene.
Living rubbery polymers can also be endlinked with tin halides or silicon halides. For instance, it is common in the art to couple living rubbery polymers with a stoichiometric amount of tin tetrachloride or silicon tetrachloride. The coupling of a lithium terminated polymer with silicon tetrachloride is illustrated in the following reaction scheme:
wherein P represents polymer chains. As can be seen one mole of silicon tetrachloride is required for every four moles of lithium terminated polymer chains. In other words, one mole of silicon tetrachloride is required for every four moles of lithium in the lithium terminated polymer being treated. This relationship must be stoichiometrically perfect in order to endlink every lithium terminated polymer chain in the polymer being treated. This is because if too much (more than a stoichiometric amount) of silicon tetrachloride is utilized the polymer chains can react with the silicon tetrachloride to become terminated with a chlorinated silicon moiety without being coupled to another polymer chain. Such unbranched polymer chains have the structural formula:
wherein P represents polymer chains. Such silicon trichloride terminated polymer chains will not react with each other to facilitate coupling.
Due to the fact that the amount of silicon trichloride needed to endlink a metal terminated polymer must be stoichiometrically perfect in order to maximize the endlinking of such polymers, it is virtually impossible to endlink 100 percent of the metal terminated polymer chains in a polymer by utilizing silicon tetrachloride as the crosslinking agent. Another drawback associated with utilizing silicon halides as crosslinking agents is that a maximum of four polymer chains can be endlinked together in each polymer network formed.
U.S. Pat. No. 4,618,650 offers a process for endlinking metal terminated polymers wherein a molar excess of the crosslinking agent can be utilized without maintaining a perfect stoichiometric relationship in order to attain maximum crosslinking. The utilization of this technique can also result in the formation of huge polymeric networks which can theoretically contain an infinite number of polymer chains.
U.S. Pat. No. 4,618,650 discloses a process for endlinking a metal terminated polymer comprising: (a) reacting the metal terminated polymer with a molar excess of a halogenated silicon containing compound, wherein the halogenated silicon containing compound contains at least two halogen atoms which are bonded directly to a silicon atom, to produce a polymer which is terminated with halogenated silicon moieties wherein the halogenated silicon moieties contain at least one halogen atom which is bonded directly to a silicon atom; (b) reacting the polymer which is terminated with halogenated silicon moieties with a molar excess of a tertiary alcohol to produce a polymer which is terminated with hydroxy silyl moieties; and (c) allowing the polymer which is terminated with the hydroxy silyl moieties to endlink under conditions sufficient to produce a network polymer containing siloxane linkages.
U.S. Pat. No. 4,618,650 more specifically discloses a process for endlinking a metal terminated polymer comprising: (a) reacting the metal terminated polymer with a molar excess of a halogenated silicon containing compound having the structural formula
wherein X represents a halogen and wherein Z1 and Z2 can be the same or different and are selected from the group consisting of halogens and alkyl groups, to produce a polymer which is terminated with halogenated silicon moieties, wherein the halogenated silicon moieties have the structural formula
wherein X represents a halogen and wherein Z1 and Z2 can be the same or different and are selected from the group consisting of halogens and alkyl groups; (b) reacting the polymer which is terminated with halogenated silicon moieties with a molar excess of a tertiary alcohol to produce a polymer which is terminated with hydroxy silyl moieties, wherein the hydroxy silyl moieties have the structural formula
wherein A1 and A2 can be the same or different and are selected from the group consisting of alkyl groups and hydroxyl groups; and (c) allowing the polymer which is terminated with the hydroxy silyl moieties to endlink under conditions sufficient to produce a network polymer containing siloxane linkages.
The network polymers formed by the process of U.S. Pat. No. 4,618,650 are usually the reaction product of one or more polymers which are comprised of polymer chains which are terminated with at least one moiety which has a structural formula selected from the group consisting of
wherein R1 and R2 are alkyl moieties which can be the same or different. The network polymers made by the process of U.S. Pat. No. 4,618,650 are essentially in the form of stars with the polymer chains being coupled together at a point near the center of the star.